The present invention relates generally to subscription television signal decoders and, more particularly, to an improved decoder for unscrambling a sine-wave encoded television signal.
In subscription television systems, television signals are typically transmitted to system subscribers in a scrambled or encoded form either along a coaxial cable or as an "over-the-air" broadcast. Each system subscriber is provided with a decoder operable for unscrambling or decoding the transmitted television signal and for supplying the unscrambled signal to a standard television receiver for viewing.
A relatively common technique for scrambling or encoding a subscription television signal contemplates the application of a sine wave signal as additional amplitude modulation to the composite television signal. The frequency of the encoding sine wave signal is normally equal to the horizontal scan rate (approximately 15.75 KHz), although other integral multiples thereof are sometimes used, and is phased locked to the horizontal sync such that the resulting sine wave modulation has the effect of suppressing horizontal sync and enhancing the video between horizontal sync pulses. U.S. Pat. Nos. 3,081,376 to Loughlin et al, 3,729,576 to Court and 4,024,575 to Harney et al are exemplary of such subscription television sine wave encoding systems.
In order to decode the transmitted sine wave encoded television signal, it is known to amplitude modulate the audio carrier of the television signal with a complementary decoding sine wave signal. The decoding sine wave signal is recovered from the audio carrier by the decoder and applied to the video signal to cancel the encoding sine wave amplitude modulation for reconstituting the television signal in an unscrambled form. For example, in the referent Loughlin et al patent, the decoding sine wave signal is recovered from the audio carrier by a narrow band detector and applied to an RF modulator for cancelling the encoding sine wave modulation from the video signal. Similarly, in the Court patent the decoding sine wave signal is recovered from the audio carrier by narrow band intermediate frequency filters and applied through a 15.75 KHz tuned amplifier to a decoding modulator for cancelling the encoding modulation. In the Harney et al system, the decoding sine wave signal is recovered from the residual modulation characterizing the audio carrier after it and the video carrier have been processed by an RF amplifier whose gain is controlled by the decoding signal.
All of the foregoing prior art systems are characterized in that the decoding signal is recovered from amplitude modulation of the audio carrier. This common prior art approach suffers from the disadvantage that relatively expensive and critically designed audio recovery circuits are necessary to recover the decoding signal from the audio carrier. In addition, increased circuit complexity and expense are required at the transmitter to properly modulate the audio carrier with the correct phase decoding signal.
It is accordingly a basic object of the present invention to provide an improved decoder for decoding or unscrambling a sine wave encoded television signal.
It is a more specific object of the invention to provide a decoder for decoding a sine wave encoded television signal wherein a decoding sine wave signal is derived independently of the audio carrier.